Electronic pick-up tube for incident x-rays with image intensifier



April 1969 J. FINKLE ELECTRONIC PICK-UP TUBE FOR INCIDENT X-RAYS WITHIMAGE INTENSIFIER Filed Sept. 5, 1963 Attorney .LNHEWdS V31. N43 -XUnited States Patent US. Cl. 250213 8 Claims ABSTRACT OF THE DISCLOSUREImage intensifier for electronic pick-up tube in which X-rays impingeupon a fluorescent layer whose luminous output stimulates the emissionof electrons from a photocathode to generate an electron beam having thepattern of the incident radiation, the intensifier including aphotoelectro-luminescent phosphor layer and a photoconductive layer withan intervening barrier layer between a pair of conductive layersconnected across an alternating-voltage source; the barrier layer isopaque to visible light but transparent to the incident X-rays so thatthe latter excite the adjoining phosphor layer, this excitation beingreinforced by the concurrently applied alternating field which varieslocally in conformity with the conductivity pattern developed in thephotoconductive layer on the other side of the barrier.

My present invention relates to an electronic pick-up tube designed toproduce an intensified visible picture of an incident X-ray image,either by direct conversion or upon interim preservation on a storagemedium.

It is known to convert a pattern of incident X-rays into acorrespondingly patterned electron beam by letting the X-rays impingeupon a fluorescent layer whose photons excite an electron-emissivelayer; it is also known to convert an X-ray image into an intensifiedvisible image by applying a voltage across a multilayer structureincluding a photoconductive layer and an electroluminescent layerseparated by a high-resistance barrier, the photoconductive substancebeing stimulated by the incident radiation.

The general object of my invention is to provide, in a system forvisualizing X-ray images by electronic means, an improved electronicimage intensifier allowing the use of reduced doses of radiation whilegiving a clear picture for diagnostic purposes.

I have found, in conformity with my present invention, that this objectcan be realized by utilizing, as part of an image-intensifier structureof the general character referred to, a barrier layer which issubstantially opaque to luminous radiation from a preceding fluorescentlayer but is transparent to the same X-rays which excite the fluorescentlayer to generate a conductivity pattern in a photoconductive layer onone side of the barrier, the combined effect of this conductivitypattern and of the penetrating incident radiation acting upon aphotoelectroluminescent layer on the other side of the barrier togenerate a bundle of photons directed onto an adjoiningelectron-emissive layer inside a tube envelope provided with outputmeans (such as a viewing screen or a scanning system) for making theresulting beam pattern visible with or without intermediate storage.

According to a more specific feature of my invention, thephotoconductive, barrier and photoelectroluminescent layers of anintensifier structure as described above are subjected to an alternatingelectric field from a voltage source connected across a pair ofradiation-permeable conductive layers. The magnitude of this alternatingfield varies, in a direction perpendicular to the tube axis, accordingto the pattern of conductivity generated in the photoconductive layer bythe incident radiation. Since the areas of locally increased fieldstrength coincide with the regions of concentrated X-rays impingingdirectly upon the photoelectroluminescent layer, the rate of photonemission from the latter is greatly enhanced. Thus, the incident X-raysneed only be strong enough to activate the irradiated phosphor of thephotoelectroluminescent layer, the energy required for the amplificationof the photon output being obtained from the applied electrostaticfield. The adjacent barrier layer, acting as a mirror for light emittedby the photoelectroluminescent phosphor layer, insures that all of thislight is directed toward an electron-emissive layer disposed beyond thephosphor layer.

The phenomenon of photoelectroluminescence manifests itself in thefollowing manner:

When incident radiation falls upon a phosphor layer exhibiting thisproperty, an emission of visible light occurs in the absence of anelectric field. The application of a voltage across the phosphor layerreduces the emission of light, yet subsequent removal of this quenchingvoltage produces a sudden flash followed by a return to the originalsteady-state intensity. Thus, the application of an alternating voltageacross the barrier layer results in repeated charges and discharges ofthe phosphor layer with emission of intense luminescent radiation,appearing to the eye as a continuous illumination, from the X-ray-'stimulated areas thereof.

The electron-emissive layer, together with the preceding fluorescentlayer and the intervening intensifier structure, constitutes thephotocathode of an otherwise conventional electron tube which may beequipped with an electronexcitable luminescent screen to reveal avisible picture of the radiation image. Alternatively, and in accordancewith a preferred embodiment, the tube is provided with scanning means ofthe type used in television camera tubes, such as those of the Vidicon,image-orthicon or image dissector (Farnsworth) type. The output of sucha tube may then be temporarily preserved in a storage tube forsubsequent reproduction.

The invention will be described in greater detail hereinafter withreference to the accompanying drawing the sole figure of which is asomewhat diagrammatic illustration of a representative embodiment.

As shown in the drawing, a Farnsworth-type tube 2 having the usualevacuated envelope is provided with a composite photocathode positionedto receive an incident X-ray pattern 1. This photocathode comprises amultiplicity of layers transparent to X-rays, namely a lightreflectingsupporting layer 3, a fluorescent layer 4 carried by reflector 3, a baseplate 5, a conductive layer 6, a photoconductive layer 7, and ahigh-resistance barrier layer 8 opaque to visible light. Layer 8 carriesa photoelectroluminescent phosphor layer 9 adjoining another conductivelayer 10 which is transparent to luminescent radiation emitted by layer9. Electrodes 6 and 10 may comprise an easily volatilizing metal, suchas aluminum, silver or gold, deposited in vacuo with a thickness ofabout 5 to 10 mils. An electron-emissive layer 11, forwardly of thestack of layers 5-10, generates an electron beam 16 in response to lightemitted by phosphor layer 9; layer 11 may consist of material alsoresponsive to incident X-rays, e.g. bismuth, as disclosed in US. PatentNo. 2,897,388, in which case the rate of electron emission is furtherincreased.

The remaining, conventional elements of tube 2 include a focusing coil12 surrounding its envelope, a set of accelerating anodes 13a, 13b, 13c,13d of progressively increasing axial widths and progressively higherpositive potential obtained from a common potentiometer 13, anelectron-multiplier structure 17 with entrance orifice 17' for the beam16, scanning electrodes (partly shown at 14, 15) for deflecting the beamacross this orifice, and a target electrode 18 aligned with the orifice,the latter electrode forming part of an output circuit not furtherillustrated.

A generator 21 of alternating current is connected across the two layerelectrodes 6, 10 by way of a transformer 31 in series with a switch 32,the secondary of this transformer being connected in series with acondenser 29 which bridges a biasing battery 23 and an associatedcircuit breaker 24. The polarity of battery 23 is so chosen thatelectrode 10, and therefore also the adjoining phosphor layer 9, is morehighly positive than electrode 6 and adjoining photoconductive layer 7upon closure of circuit breaker 24.

A source of variable biasing voltage 33 is shown connected to thebarrier layer 8 which may thus be utilized in the manner of a controlgrid to vary the intensity of the image.

The magnitude of the alternating field developed across electrodes 6 and10 may range between 600 and 1000 volts; if layer 9 has a thickness ofabout 100 microns, the average field strength within the layer can bebetween about 10 and 10 volts per centimeter. A preferred frequency is800 cycles/sec.

The potential of the electron-emissive layer 11 may be suitably selectedby conventional biasing means not illustrated.

The phosphor layer 9 may consist of organic or inorganic material, e.g.cesium iodide imbedded in a plastic coating; suitable organic materialsinclude such polycyclic compounds as stilbene, anthracene, naphthaleneand phenanthrene combined with calcium-titanium silicate,calcium-titanium stannate, co-crystallization products of lanthanumsilicate, cerium oxide, calcium orthophosphate/stannate, and calciumorthophosphate/cerium oxide. Favorable emission spectra are obtainedwith calcium tungstate admixed with lead tungstate and a halogen. Othersuitable' compounds are zinc silicate, zinc selenide, zinc sulphide,barium-lead sulphate, zinc-cadmium sulphide, magnesium tungstate andzinc-beryllium silicate, combined with a halogen and/or some otheractivator. The inclusion of a small amount of lead in the phosphor layerincreases its sensitivity to X-rays. An admixture of one mole percent ofcopper with calcium tungstate will cause a 56% increase in the intensityof photon emission upon excitation by X-rays. With silicates there maybe added a titanium activator in a molar ratio of 1:10. Advantageously,the radiation emitted by layer 9 should be near the blue region of thespectrum (4000 to 6000 angstroms) and should correspond to the band ofgreatest sensitivity of emissive layer 11. Layer 9 should behomogeneous, continuous and nongranular and should exhibit uniformelectrical properties throughout its area; reference may be made to US.Patent No. 2,685,530 for the preparation of such layers.

If desired, the supporting layer 3 may be omitted, with fluorescentlayer 4 carried directly on the envelope of tube 2. Layer 11 may besupported on a further transparent layer, not shown, which should beconductive for a removal of the charge built up by the emission ofelectrons during operation of the device. These and other modifications,readily apparent to persons skilled in the art, are intended to beembraced within the spirit and scope of my invention as defined in theappended claims.

Having described my invention, what I desire to claim by Letters Patentis:

1. In an electronic pick-up tube for incident X-rays comprising anevacuated envelope, an electron-emissive layer responsive to incidentlight in said envelope and output means in said envelope for convertinga beam of electrons emitted by said layer into a visible picture, thecombination therewith of a fluorescent layer in the path of incidentX-rays within said envelope, and a multilayer image-intensifierstructure between said electron-emissive layer and said fluorescentlayer within said envelope, said structure including a photoconductivelayer disposed to receive luminous radiation from said fluorescentlayer, a photoelectroluminescent layer interposed between saidelectron-emissive layer and said photoconductive layer in closeproximity to the latter, and a high-resistance barrier layer betweensaid photoconductive and photoelectroluminescent layers, said barrierlayer being substantially opaque to said luminous radiation buttransparent to X- rays penetrating said fluorescent layer for enablingexcitation of said photoelectroluminescent layer jointly by saidincident X-rays and by an electric field emanating from saidphotoconductive layer, thereby directing upon said electron-emissivelayer a bundle of photons in the pattern of said incident X-rays togenerate a corresponding electron beam.

2. The combination defined in claim 1 wherein said structure includes apair of radiation-permeable conductive layers bracketing saidphotoconductive and photoelectroluminescent layers, further comprising asource of alternating voltage connected across said conductive layers.

3. The combination defined in claim 2, further comprising a biasingcircuit in parallel with said source for impressing upon saidphotoelectroluminescent layer a D-C potential more positive than that ofsaid photoconductive layer.

4. The combination defined in claim 2 wherein said source has an outputvoltage suflicient to apply to said electron-emissive layer analternating field of a magnitude of substantially 10 to 10 volts per cm.thickness.

5. The combination defined in claim 2 wherein said source has afrequency on the order of 800 cycles per second.

6. The combination defined in claim 1 wherein said barrierlayer isprovided with means for applying thereto a biasing potential controllingthe degree of excitation of said photoelectroluminescent layer by saidphotoconductive layer.

7. The combination defined in claim 1 wherein said electron-emissivelayer consists of a material excitable by said incident X-rays andpositioned to receive said X-rays concurrently with said bundle ofphotons.

8. The combination defined in claim 1 wherein said barrier layer is areflector for luminous radiation gen erated in saidphotoelectroluminescent layer.

References Cited UNITED STATES PATENTS 3,064,133 11/1962 Murr et al250-213 2,593,925 4/1952 Sheldon 250-213 2,929,935 3/1960 Lempert250-213 3,073,989 1/1963 Amsterdam 250-213 3,210,551 10/1965 Vaughn eta1. 250-213 RALPH G. NILSON, Primary Examiner.

MARTIN ABRAMSON, Assistant Examiner.

